Area Forecast Discussion
Issued by NWS Greer, SC

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000
FXUS62 KGSP 220556
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
156 AM EDT Sun Apr 22 2018

.SYNOPSIS...
Dry high pressure over the eastern United States will maintain
control over the region through early Sunday morning.  Precip
chances gradually increase during the day on Sunday ahead of a
strong and slow-moving low pressure system over the lower
Mississippi River Valley.  A cold rainfall will become widespread
Sunday night into Monday and persist through early Tuesday,
resulting in significant accumulations.  The remainder of the work
week may remain unsettled due to a series of cold fronts.

&&

.NEAR TERM /THROUGH TODAY/...
As of 155 AM Sunday: Main update for the 06Z TAF issuance along
with minor tweaks made to temperatures and sky to account for
latest trends, as the rest of the near term forecast remains on
track.

With sfc high pressure well east of the area this morning, cirrus
continues to infiltrate in, gradually becoming more opaque, ahead of
the approaching system progged to bring rainfall to the area
beginning later today. With light winds and increased cloud cover,
current temperatures are mainly in the low to upper 50s, cooler
across the mountains. Do expect temperatures will continue to drop
through the early morning hours down into the mid to upper 40s.

Otherwise, gradual low-level moistening will continue overnight as a
healthy Atlantic/Gulf moisture tap sets up. An upper low and surface
low pressure system will emerge from the Deep South tomorrow,
beginning to approach the area by the late afternoon. At the same
time, a wedge of surface high pressure will slide down the spine of
the southern Appalachians, setting the stage for what will likely
become a prolonged cold-air damming event across the area in the
short term period. Skies will become progressively cloudier this
afternoon as deeper moisture enters the Carolinas, though held off
on spreading pops into the forecast area during this near-term
period except for our north GA counties and far southwestern Upstate
SC. Highs temperatures today are expected to be about a category
below average due to increased cloud cover.

&&

.SHORT TERM /TONIGHT THROUGH TUESDAY NIGHT/...
As of 245 PM Saturday: The first part of next week still looks quite
wet across the region, and this still looks like a troublesome
fcst with lots of places that it could go wrong. Fortunately,
the model guidance remains in generally good agreement with the
overall wet pattern, that being a large upper low moving slowly
east from the MS River valley Sunday night and then lingering over
the TN valley Monday and Monday night. We should be well on our
way to establishing a cold air damming regime at sunset on Sunday
with light precip breaking out from SW to NE to seal the deal in
the evening, given a complex parent sfc high stretching from ern
Ontario to the Mid-Atlantic coast supplying the dry NE boundary
layer flow and strengthening SE moist upslope/isentropic lift over
top of it. Confidence is high that the entire fcst area will have
rain Monday and Monday night given the overwhelming support of
very strong low level isentropic upglide, deep moisture, and deep
forcing as the upper low spins over the TN valley. Rainfall amounts
are the important part of the fcst. Expect the heaviest precip to
develop across mainly the Savannah R basin, Upstate SC, and the
SE upslope areas into the upper French Broad R basin by Monday
daybreak, and then persist in that region as we have something of
a conveyor belt of Atlantic moisture focused on that region into
Monday night. A few inches of rain is likely, at least 2-3 inches,
with higher amts in the usual upslope region. Whether or not this
will be sufficient to result in flooding is questionable as we
may lack the sfc-based convection to really jack up the precip
rates that would cause problems on the small streams. Instead,
it looks more like a duration-type issue, in which case the
runoff will probably not pile up enough to cause problems until
late in the day on Monday. Confidence is not quite high enough
to entertain a Watch for an event that would mainly be in the
4th fcst period, so we will hold off for the time being. Wind
will also be strong, but mainly a problem above about 5k feet,
so a High Wind Watch will also be tabled. Temps will most likely
not make it out of the 50s east of the Blue Ridge with the cold
air damming in place and extensive precip. Expect a low diurnal
range. Tuesday and Tuesday night should see the upper low start
to open up and then get kicked up the OH River valley, so the
demise of the CAD regime becomes problematic. Guidance suggests
that isentropic lift and warm advection will taper off on Tuesday
and deeper moisture will get stripped away as a dry slot attempts
to wrap in from the SW. However, it is one thing to remove the
reinforcement for the wedge and another thing to bring in some
destructive agent. The NAM kills off the CAD during the day as a
triple-point low moves through, but really doesn`t show much of
any other destructive signal. The NAM would be a problem because
it would allow a plume of sfc-based CAPE greater than 1000 J/kg to
advect into the wrn Piedmont in the afternoon with enough residual
shear to organize severe thunderstorms. The GFS, meanwhile, keeps
a low level easterly flow longer into the day which would support
holding onto some remnant cool pool in the usual damming region
into the afternoon. The GFS seems the more reasonable solution
and agrees with the SREF mean muCAPE. That being said, even the
GFS suggests caution in that enough destabilization could occur up
into, say, the CLT metro area in the afternoon with sbCAPE upwards
of 500 J/kg, with a remnant wedge boundary hanging out in the I-85
corridor. At this point, we will keep this idea on the back burner,
as one should at this time of the year. The forcing and precip
should lift out to the NE in the evening ending any threat for
heavy precip/severe storms. Think we will develop something of a
lull early Wednesday before the next system approaches, save some
remaining chance of light precip on the TN border.

&&

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
As of 230 PM Saturday: A series of short waves will move through the
long wave upper trough over the eastern CONUS through the period.
However, there is considerable disagreement on timing and location
of these systems from model to model and from run to run. This makes
for a low confidence forecast each day. Therefore, have gone with a
model blend due to these discrepancies and the low confidence. Looks
like generally diurnal PoP trends Wednesday and Thursday, with best
chances across the mountains, as moisture lingers and a series of
weak waves and a cold front cross the area. The GFS shows another
weak wave and cold front moving through on Friday with mainly low
end diurnal PoP while the ECMWF shows it on Saturday. Have gone with
the GFS and Canadian for Friday and Saturday given their agreement.
Highs start out near normal, drop a few degrees for Thursday, then
bounce back to just below normal for Friday and Saturday. Lows start
out a few degrees above normal then remain steady near normal
through the end of the period.

&&

.AVIATION /06Z SUNDAY THROUGH THURSDAY/...
At KCLT and elsewhere: VFR prevails across the area this morning
with cirrus infiltrating in and becoming more opaque ahead of an
approaching system. With the exception of KAND where MVFR cigs and -
SHRA could move in as early as 15Z, expect VFR cigs to become
widespread through early this afternoon with -SHRA and the
deterioration to MVFR cigs becoming widespread by this evening.
MVFR/IFR cigs will persist through the end of the valid TAF period,
with ongoing -SHRA. Light winds this morning are expected to
increase to 5 to 10 kts, becoming NE at KGSP/KGMU/KAND due to
developing cold air damming, and remaining ESE at the other TAF
sites. Intermittent gusts up to 20 kts could not be entirely ruled
out this evening.

Outlook: Conditions will deteriorate Sunday night with widespread
rain overspreading the area. IFR/MVFR conditions will be likely thru
the day on Monday, with some improvement on Tuesday, as precip
largely shifts NE of the area, but with some moisture lingering.
Unsettled weather will continue on Wednesday and Thursday, as a
trough of low pressure persists across the region.

Confidence Table...

            06-12Z        12-18Z        18-24Z        00-06Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High  80%
KAVL       High 100%     High  97%     High  99%     High  90%
KHKY       High 100%     High 100%     High 100%     High  86%
KGMU       High 100%     High 100%     High 100%     Med   77%
KAND       High 100%     High  96%     High  94%     High  80%

The percentage reflects the number of guidance members agreeing
with the scheduled TAF issuance flight rule category. Complete hourly
experimental aviation forecast consistency tables and ensemble forecasts
are available at the following link:

www.weather.gov/gsp/aviation

&&

.HYDROLOGY...
Due to the predominant environment, rain rates associated with the
upcoming event are generally NOT expected to be supportive of flash
flooding, where flooding begins within 6 hours of rainfall onset
and/or initial stream response.  Model guidance has consistently
kept 6-hourly rain accumulations generally under 1.5", which lessens
the concern for significant (i.e., flooding of primary roadways
and structures) flooding.  Instead, a more gradual accumulation
of significant rainfall means that any flood threat will gradually
increase Monday afternoon into Tuesday as widespread soil saturation
is reached and persistent runoff over large areas causes significant
rises on area streams.  Low-lying areas that commonly flood due
to poor drainage and overwhelmed drainage ditches will be most
susceptible and such flooding may persist for many hours due to
the steady and persistent nature of the rainfall.

As a result, minor (i.e., parks, farmland, boat access areas,
and other common flood-prone low lying areas) flooding of the
Upper French Broad River and associated tributaries is possible,
but minor flooding of mainstems in the Upstate is also possible,
especially along the Broad and Saluda rivers, with crests not
occurring until Tuesday and perhaps early Wednesday.  Streamlevel
forecasts for these rivers will incorporate the total forecasted
rainfall beginning Sunday morning and will be accessible at
http:/water.weather.gov/ahps2/index.php?wfo=gsp.

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.

&&

$$
SYNOPSIS...JMP
NEAR TERM...Carroll/SGL
SHORT TERM...PM
LONG TERM...RWH
AVIATION...SGL
HYDROLOGY...JMP



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